Air-operated self-propelled percussive cyclic action machines for making holes in the soil are known. These machines comprise a hollow cylindrical housing, having a pointed head section, a striker which reciprocates inside the housing, and an air-distributing mechanism. The principle of operation of these machines is as follows. During one cycle of machine operation the striker executes a forward and then a backward stroke. At the end of the forward stroke the striker, being accelerated by compressed air, imparts an impact to the front end of the housing. As a result, the machine penetrates the soil by a certain increment of displacement. During the backward stroke the striker is braked, e.g., by an air buffer which develops in the space between the rear end of the striker and the housing. The air buffer prevents a collision between the striker and the housing, so that the striker stops and a new cycle begins. Machines of this type are described in Zinkiewicz, U.S. Pat. No. 3,137,483 and Zygmunt, U.S. Pat. No. 3,407,884.
A number of inherent disadvantages have prevented the extensive utilization of these machines. One of the main shortcomings of these machines is the insufficient reliability of the air-distributing mechanism. Improved versions of these machines, based on a valveless air-distributing system, were later developed. The valveless air-distribution system described in U.K. Patent No. 800,725, published Sep. 3, 1958 was later used in a soil penetrating machine described in U.S. Pat. No. 3,410,354, issued to Sudnishnikov et al. in November, 1968. U.S. Pat. No. 3,651,874, issued to Sudnishnikov et al. in March, 1972, described a reversible valveless machine for making holes in the soil. This machine provided a threaded connection between the air supply sleeve and the machine body, allowing the stroke of the striker to be displaced rearwardly.
Subsequent patents illustrate that the valveless machines of Sudnishnikov et al. suffered from various problems. U.S. Pat. No. 3,708,023 issued to Nazarov et al. in January, 1973, noted that prior self-propelled machines did not possess sufficient impact power, and proposed to solve the problem by providing an auxiliary pressure chamber at the head of the machine. U.S. Pat. No. 3,727,701, issued to Sudnishnikov et al in April, 1973, mentions that, in known reversible air-punching machines for making holes in the soil, pressure fluctuations may result in an uncontrollable shifting of the machine from the forward to reverse mode, and vise versa.
In U.S. Pat. No. 3,744,576, issued to Sudnishnikov et al. in July, 1973, it is stated that the screw reversing mechanisms of known pneumopercussive machines, which require rotating the air supply hose to displace the stroke of the striker, are difficult and sometimes impossible to use. Nonetheless, Sudnishnikov et al., U.S. Pat. No. 3,756,328, issued September 1973, still shows a screw reversing mechanism which must be manually actuated by rotating the air hose. The '328 patent further describes a resilient shock-damping means having longitudinal air exhaust passages designed to prevent from early breakdown of the air-distributing mechanism. Machines based on U.S. Pat. No. 3,756,328 are still in use.
Subsequent patents describe a variety of largely unsuccessful attempts to improve the reversing mechanism of valveless soil penetrating machines. Sudnishnikov et al., U.S. Pat. No. 4,078,619, issued in March, 1978, discloses an improvement to the reversing mechanism. According to this patent, the reversing mechanism is actuated by manually pulling on the air supply hose. Tkach et al., U.S. Pat. No. 4,121,672, issued in October, 1978, offers an improvement to the means for rotating the air supply hose. U.S. Pat. No. 4,132,277, issued to Tupitsyn et al. in January, 1979, also describes a reversing mechanism which is activated by pulling the air supply hose. U.S. Pat. No. 4,214,638 issued July 1980 to Sudnishnikov et al., states that controlling the reverse mechanism by rotating or pulling the air supply hose is time consuming, difficult and, in certain cases, altogether impossible. This has proven true in practice particularly when it is necessary to reverse the machine when the machine is far underground.
To address these problems, Schmidt, U.S. Pat. No. 4,295,533, issued October, 1981, suggests rotating a component in the reversing mechanism by a flexible shaft enclosed within the air supply hose. Bouplon, U.S. Pat. No. 4,662,457, issued May, 1987, offers an improved reversing mechanism which requires rotating the air-supplying hose approximately a quarter of a turn. U.S. Pat. No. 4,683,960, issued August, 1987, to Kostylev et al., describes a reversing mechanism based on pulling a separate cable instead of the air-supplying hose. None of these manually-operable reversing mechanisms have completely eliminated the problems with the screw reverse mechanism.
A different approach to control the reversing mechanism is proposed in Schmidt, U.S. Pat. No. 4,250,972, issued February, 1981. According to this patent, the reversing mechanism is controlled by a secondary air supply line to the device, or electrically. However, this system has not found widespread use.
This brief analysis of the valveless pneumo-percussive underground penetrating machines shows that, during the last two decades, many unsuccessful efforts were made to improve the control system of the reversing mechanism for underground penetrating machines. However, despite these efforts, the basic screw reverse operated by rotating the air supply hose a dozen times or more remains in widespread use.
Other improvements to the valveless soil penetrating machine of U.S. Pat. No. 3,410,354 have also been proposed Schmidt, U.S. Pat. No. 3,865,200, issued February, 1975, describes a movable chisel and an intermediate piston having interposed elastic members. This patent asserts that this reduces impact loading on the housing of the penetrating machine, and also reduces the required "percussion energy" in comparison with conventional driving machines. Although this design does reduce impact loading on the housing, there is an energy loss due to the intermediate piston. In addition, the movable chisel is ineffective because, after a number of working cycles, particles of soil penetrate fill the gap between the chisel and the housing, so that the chisel becomes jammed in the forwardmost position. The intermediate piston and movable chisel disappeared from the later machines by the same inventor (See Schmidt, U.S. Pat. Nos. 4,250,972 and 4,295,533). A movable chisel is also shown in the U.S. Pat. No. 4,100,980 issued to Jenne in July, 1978. This design is also unworkable for the same reasons as in the Schmidt device.
Energy consumption and productivity are among the most important parameters of a working process of a machine. During the last decade several scientific papers, related to the energy consumption and productivity (or average velocity) of underground percussive penetrating machines have been published by the present inventor. See Minimization of Energy Consumption of Soil Deformation, Journal of Terramechanics, 1980, Volume 17, Number 2, pages 63 to 77; Principles of Soil-Tool Interaction, Journal of Terramechanics, 1981, Volume 18, Number 1, Pages 51 to 65; Motion of Soil-Working Tool Under Impact Loading, Journal of Terramechanics, 1981, Volume 18, Number 3, Pages 133 to 156; Working Process of Cyclic-Action Machinery for Soil Deformation-Part 1, Journal of Terramechanics, 1983, Volume 20, Number 1, Pages 13 to 41; Minimum Energy Consumption of Soil Working Cyclic Processes, Journal of Terramechanics, 1987, Volume 24, Number 1, Pages 95 to 107). According to the data presented in these papers, the process of vibratory soil penetration can be optimized to obtain minimum energy consumption. By comparing the performance of the conventional pneumopercussive hole making machines with the performance possible using minimum energy consumption, it becomes clear that conventional machines are characterized by relatively high energy consumption and relatively low productivity. Development of new machines based on optimization with respect to minimum energy consumption will decrease the flow rate of the compressed air and also simultaneously increase the average velocity of these machines.
To optimize the boring process, it is essential to increase the kinetic energy that the housing obtains from an impact of the striker. One way to increase this kinetic energy consists of lengthening the forward stroke of the striker. However, the structure of the valveless air-distributing mechanism of known pneumopercussive underground penetrating machines makes it very difficult or almost impossible to increase the striker stroke length to a considerable extent. The reason is that the backward stroke of the striker occurs under the action of a portion of the compressed air which enters the rear stroke chamber through holes that remain open a short time. These holes are then closed by overlap of the striker during the beginning of its backward stroke. Pressure force of the expanding air moves the striker backward. In the forward stroke chamber there is constant air pressure. The striker moves backward because the active, cross-sectional area of the striker for the backward stroke exceeds the active cross-sectional area for the forward stroke. However, the backward stroke cannot be relatively long because the pressure of the expanding air in the backward stroke chamber air drops rapidly as the pressure in the forward stroke chamber brakes the striker. Thus, the valveless air-distributing mechanisms of the type discussed above inherently require relatively short striker stroke lengths. The valveless air-distributing mechanism of conventional machines is not appropriate for relatively long stroke machines, for example, 1.5 to 2 times.
Another inherent disadvantage of conventional pneumopercussive underground boring machines is that the backward stroke chamber is connected with the atmosphere for just a brief period during the forward stroke of the striker. This creates an air buffer in the backward stroke chamber and brakes the striker before it imparts an impact, thereby decreasing the kinetic energy of the striker before impact. The auxiliary chamber proposed in the foregoing patent to Nazarov et al. has not proven an effective solution to this problem.
A further disadvantage of known pneumo-percussive underground machines concerns the ratio between the external frictional force of the soil distributed over the surface of the housing and the rearward air pressure force applied to the inside rear end of the housing during the forward stroke of the striker, i.e., the recoil of the housing as the striker moves forward. Under working conditions this ratio is in the range from 0.3 to 0.75, while the optimum value this ratio, as taught in the literature, is 1.0. This means that, under actual working conditions, the housing moves backward during the forward stroke of the striker. Such movement negates a certain part of the stroke length, and the housing gains a negative velocity before the collision. The striker cannot actually utilize the entire stroke length and, consequently, has less kinetic energy before collision. The decreased kinetic energy of the striker and the backward velocity of the housing before collision in turn reduce the kinetic energy of the housing after the collision, reducing the overall efficiency of the machine.
Still another inherent disadvantage of known pneumopercussive underground penetrating machines is associated with the transmission of kinetic energy from the striker to the housing. The best situation is when the rebound energy of the striker is equal to zero so that the housing gains the maximum possible energy from the striker. Collision theory dictates that, in order to obtain ideal transfer of energy from the colliding mass to a motionless collided mass, the ratio between the colliding mass and the collided mass must equal the value of the restitution coefficient of the two masses. However, in conventional pneumopercussive underground penetrating machines, the ratio of these masses is significantly less than the restitution coefficient. This causes the striker to rebound so that part of the kinetic energy of the striker is not transferred to the housing. Ideally, the striker should stop dead in the same way a billiard ball does when striking another ball.
Still another inherent disadvantage of conventional pneumopercussive underground hole making machines is the lack of a means for independently controlling the compressed air in the forward and backward stroke chambers. Under some working conditions the striker can impart undesirable impacts to the rear end of the housing. These impacts can be avoided by controlling the air pressure in the backward stroke chamber. Similarly, when the machine is in reverse mode it may be necessary to control the pressure of the compressed air in the forward stroke chamber to prevent forward impacts. Independent control of the compressed air in the forward and backward stroke chambers can also improve the efficiency and restarting ability of the machine.
One more inherent disadvantage of conventional pneumopercussive underground penetrating machines is the lack of a means for monitoring the working process of the machine. The striker frequency and pressure in the forward stroke chamber change depending on operating conditions. The impact frequency and air pressure for the forward penetrating mode and the reverse mode of the machine are different. The impact frequency also changes, e.g., when the machine meets an obstacle, or when the machine works in loosened soil. If the operator could be aware of these changes during operation, he or she could make appropriate decisions, for instance, when to reverse the machine when it meets an obstacle. In practice, these changes cannot be recognized by simply listening to the machine, and thus conventional boring machines lack any effective monitoring system.
Still another disadvantage of conventional pneumopercussive boring machines is the high impact loading that the housing experiences during operation. Severe fatigue appears in the housing that considerably decreases its service life.
The present invention addresses these disadvantages, making it possible to increase the efficiency of the machine to a considerable extent.